Electric Fields
Electric Fields Forces on charges * Electric charge Q ''' is measured in '''coulombs © and can be either positive (carried by particles like protons) or negative (carried by particles like electrons) * The smallest unit of charge is 1.6 × 10^-19C (the size of the charge on an electron or proton) * Oppositely charged particles attract each other. Like charges repel.(The force experienced is given by Coulomb's Law) Calculate forces using Coulomb's Law * F=kQ1Q2/r^ 2 * F''' is the force on either charge (N) * '''Q1 and Q2 '''are the two charges © * '''r is the separation of the centres of the two charges (m) * k''' is a constant; its value is '''9 × 10^9 Nm^2 C^-2 * k = 1/(4πε0 ), where ε0 is the permittivity of free space. Electric Field Strength * Electric field strength ,E''', is defined as the '''force per unit positive charge.(It's the force that a charge +1 C would experience if it was placed in the electric field.) * E=F/Q ''' Where '''E is electric field strength(NC^-1)， F''' is the force(N) and '''Q is the charge © * Note that since force is a vector and charge is a scalar, electric field strength must be a vector.(pointing in the direction that a positive charge would move) Representing electric fields Fields are represented by drawing field lines (lines with arrows) * The closer together the lines are the stronger the field. * Field lines point in the direction a positive test charge would move. Types of electric field Point charges * A point charge, or any body that behaves as if all its charge is concentrated at the centre, has a radial field. The electric field for such a charge can be found from Coulomb's law F=kQq/r^2 F = qE '''This gives '''E=kQ/r^2 Parallel plate capacitor * A uniform field can be produced by connecting two parallel plates to the opposite poles of a battery * In a uniform field , the field lines are parallel so they're always the same distance apart. This means that the field strength is the same at all points within the field ( a test charge would experience the same force wherever it was.) * The field strength between two parallel plates depends on the potential difference, d''' , between them, according to the equation 'E=V/d '('''V is the potential difference between the two plates. d''' is the distance between plates. This shows that electric field strength can also have units of '''V m^-1) Variation of field with distance Point charge * The decline of E''' with '''distance follows an inverse square law (E ∝ 1/r^ 2) Parallel plate capacitor * E''' is '''constant whatever the position between the plates Electric potential * potential difference - a potential difference between two points means there is a voltage drop between them. Accordingly, electrical potential is measured in volts. * The potential at a point in an electric field is defined as the work done in bringing a test unit positive charge from infinity to that point.(The potential of a charge at infinity is always defined to be zero.) * The definition of potential in terms of work tells us that electric potential is a measure of the potential energy per unit charge(' '''The potential energy on a charge due to a potential V is: '''P.E. = qV P.E. '= potential energy (J) q''' = charge © '''V = electric potential (V) * Since energy is a scalar and charge is a scalar, potential must also be a scalar quantity. '''The work done when a charge moves through a potential difference is given by '''W = Q∆V W''' = work done (J) '''Q = charge © ∆ V = potential difference (V) * The electrical potential energy increases if a positive charge moves to a point of higher potential or a negative charge moves to a point of lower potential Potential for point charge, parallel plate capacitor * For a point charge Q V =kQ/r * For a parallel plate capacitor with spacing d''' and potential difference '''V between the plates, the potential decreases linearly between the positive and negative plates